Space/time switching elements are generally known in switching technology. By way of example, reference is made to a digital switching element operating in a synchronous time-division multiplex mode (STM) which is described in "Elektrisches Nachrichtenwesen", Vol. 59, No. 1/2, 1985, pp. 54-59, in an article by W. Frank et al., "SYSTEM 12--Doppelport des Koppelnetzbausteins". Another example is an integrated switching element ("Integriertes Koppelelement") which operates in an asynchronous time-division multiplex mode (ATM) and is described in "Elektrisches Nachrichtenwesen", Vol. 64, No. 2/3, 1990, pp. 156-165, in an article by D. Boettle et al., "Das ATM Koppelfeld von Alcatel und seine Eigenschaften".
Every development engineer endeavors to influence the ratio between cost and usefulness in a favorable manner. Constant advances in the art permit higher operating speeds, whereby capacity and, thus, usefulness can be increased nearly without additional cost. In the above-mentioned SYSTEM 12 switching system, a bit rate of 4 Mb/s (exactly 4096 kb/s) is used on the trunks. However, the current technology of that system would also permit up to 32 Mb/s. The whole construction, particularly racks, backplanes, and connectors, could remain unchanged.
One has to bear in mind, however, that in an exchange, on the one hand, a plurality of different peripheral units are interconnected by a switching network, and that, on the other hand, step-by-step expansion is desirable. It is neither possible to simultaneously redevelop all kinds of peripheral units for an increased bit rate nor desirable in case of expansion to replace existing parts. The problems associated with the procurement of replacement parts should also be taken into account.
The same problem is foreseeable for the future ATM (Asynchronous Transfer Mode) system, where already bit rates of initially 150 Mb/s and later 600 Mb/s, possibly even 1.2 Gb/s, are under discussion.